"If you are out, continue to be alert to suspicious activity,'' Patrick said. "There is still a very, very dangerous individual still at large. But we feel confident we can return to living our lives."

For most of the day, hundreds of police conducted a house-to-house sweep in Watertown for Dzhokhar Tsarnaev, 19. His 26-year-old brother, Tamerlan Tsarnaev, died early Friday morning following a dramatic car chase and shootout with police. During the chase, the brothers threw several pipe bombs and a grenade at police as they attempted to flee in a carjacked SUV.

Tamerlan, who was wounded during the shootout, was left at the scene by his younger brother, who apparently backed over him as he sped away in a carjacked Mercedes SUV.

State Police Superintendent Timothy Alben said Dzhokhar Tsarnaev later abandoned the stolen vehicle in Watertown and fled by foot. He is likely still in the Boston area, Alben said. Authorities no longer believe there are other accomplices.

"This is a dangerous person, people need to be extremely careful if they even suspect this individual,'' Alben said.

During the overnight and early-morning pursuit of the brothers, a federal official familiar with the case said authorities recovered a handful of improvised explosive devices (IEDs), including one in the possession of Tamerlan Tsarnaev. All of the devices appeared to be homemade explosives, including pipe bombs. Several were detonated by police Friday afternoon.

Police took Tamerlan Tsarnaev to Beth Israel Deaconess Medical Center following the exchange of gunfire with police at about 1:20 a.m. Friday. Dr. David Schoenfeld said medical personnel tried to revive him. He had multiple gunshot wounds and burn and gaping blast wounds that appeared to have come from an explosive device strapped to his body.

The brothers' acts continue to befuddle authorities, family and friends that know them. Students at the University of Massachusetts Dartmouth, where Dzhokar was a student, say they saw him on campus following Monday's bombings.

lnvestigators have not found any formal links so far to an international terror group.

Dzhokar's escape prompted Gov. Patrick to order the city of Boston and its surrounding suburbs locked down and its residents to remain in their homes for much of Friday. The Boston Red Sox and Bruins postponed Friday night games. Businesses in Watertown, Newton, Waltham, Belmont, Cambridge and the Alston and Brighton neighborhoods of Boston were requested to remain closed and residents to remain indoors until the suspect is caught. Massachusetts shut down all mass transit, including buses and trains, in Boston and surrounding suburbs, Kurt Schwartz, director of Massachusetts Emergency Management Agency said.

The Tsarnaev family is believed to have moved to the USA in 2003. They had lived in Kazakhstan for several years after fleeing war-torn Chechnya. Tamerlan attended Bunker Hill Community College in nearby Charlestown as a part-time student for three semesters from 2006 to 2008. He studied accounting.

In an telephone interview from Russian with the Associated Press, the brothers' father, Anzor Tsarnaev, said Dzhokhar is "a true angel" and "an intelligent boy." In subsequent media interviews, he said his sons had been framed for Monday's bombings.

Ruslan Tsarni, an uncle who had not spoken to his brother's sons since December 2005, urged Dzhokhar to turn himself in to authorities. Meeting with reporters Friday outside his home in Montgomery County, MD., Tsani said he believed the brothers may have been recently "radicalized." Tsarni says he was unaware of any military or weapons training they may have received. Tsarni called the brothers "losers" and said they had brought his family shame.

Their names were not known to law enforcement officials before the bombings, which killed three people and wounded 176. Authorities are reviewing the brothers' possible ties to Chechnya - an area of Russia plagued by Islamic insurgency - a law enforcement official who is not authorized to comment publicly told USA TODAY.

Dzhokhar Tsarnaev is believed to have dropped a backpack laden with explosives at the site of Monday's second explosion. He was pictured wearing a white baseball cap in video images released by the FBI Thursday. His page on the Russian social networking site Vkontakte says he attended Cambridge Rindge and Latin School, graduating in 2011, the year he won a $2,500 college scholarship from the city of Cambridge. On the website, his world view is described as "Islam" and he says his personal goal is "career and money."

Larry Aaronson, a neighbor and retired history teacher at Cambridge Rindge and Latin School, got to know Dzhokhar while taking photos of the high school wrestling team and other school activities.

"It's completely out of his character," Aaronson said of Dzhokhar's alleged role in the bombings. "Everything about him was wonderful. He was completely outgoing, very engaged, he loved the school. He was grateful not to be in Chechnya."

Dzhokhar was not overtly political or religious, Aaronson says. "He spoke and acted like any other high school kid."

Aaronson says he can't reconcile the young man he knows with the characterizations he's seeing in the media. "I cannot do it," he says. "I mean this from the deepest part of my heart: It's not possible it's the same person. It's just not possible."

The manhunt for the marathon bombing suspects turned into hot pursuit late Thursday evening. MIT campus police officer Sean Collier was shot multiple times as he was sitting in his car.

Collier, 26, was later pronounced dead at Massachusetts General Hospital. He had been a campus cop since January 2012 and previously, a civilian employee at the Somerville Police Department.

The suspects carjacked a Mercedes SUV between 12:15 and 12:30 a.m, holding the driver at gunpoint for a half hour before he was shoved from the car unharmed. A federal law enforcement official, who was not authorized to comment publicly, said the men allegedly told the driver that they were the Marathon bombers.

The official said the suspects' allegedly acknowledged their roles in the Marathon bombings to both intimidate the driver and brag about the bombings.

Police found the car and the suspects in Watertown, and pursued them into a residential neighborhood where gunfire was exchanged.

A transit police officer, Richard H. Donohue Jr., 33, was shot once during the firefight. He underwent surgery and is listed in critical condition at Mount Auburn Hospital.